1. Field of the Invention
The present invention relates to the preparation of adipic acid by hydrocarboxylation of pentenic acids, and, more especially, to the preparation of adipic acid by reacting water and carbon monoxide with at least one pentenic acid.
2. Description of the Prior Art
U.S. Pat. No. 3,579,551 describes the preparation of carboxylic acids by reacting ethylenically unsaturated compounds with carbon monoxide and water in the presence of a catalytic composition, essentially comprising compounds or complexes of iridium, and an iodinated promoter. The ethylenically unsaturated compounds are selectively converted into carboxylic acids (linear and branched) by conducting the reaction, preferably in liquid phase, at a temperature ranging from 50.degree. to 300.degree. C. (preferably from 125.degree. to 225.degree. C.), under carbon monoxide partial pressures advantageously ranging from 5 to 3000 psia or, more specifically, from 25 to 1000 psia.
Any source of iridium appears to be suitable and various sources of iodinated promoters are indicated; the I/Ir atomic ratio can vary over wide limits (1:1 to 2,500:1) and preferably from 3:1 to 300:1.
The liquid reaction medium can contain any solvent which is compatible with the catalytic system, C.sub.2 -C.sub.20 monocarboxylic acids being the preferred solvents.
Example 1 of this '551 patent, carried out using a propylene starting material, indicates that such a system favors the formation of branched carboxylic acids (isobutyric).
Example 19 thereof, carried out using 1-hexene, confirms the extent of the proportion of branched carboxylic acids thus obtained.
This disadvantage (lack of selectivity in respect of linear carboxylic acids) is conspicuously apparent. Indeed, in U.S. Pat. No. 3,816,489 it is proposed to conduct the subject reaction employing an I/Ir atomic ratio ranging from 3:1 to 100:1 in order predominantly to obtain terminal carboxylic acids.
Although such techniques are of considerable interest to this art, notably in the case of starting materials comprising olefinically unsaturated compounds otherwise devoid of reactive functional groups and, in particular, in the case of the olefins themselves, many difficulties are encountered in extrapolating these techniques to starting materials which contain, in addition to the site of ethylenic unsaturation, a functional group which is reactive under the conditions of the subject reaction.
Indeed, the initial attempts conducted in the laboratories of the assignee hereof to extrapolate such techniques using pentenic acid starting materials were unsuccessful, because reactions other than those intended were effected by reason of the --COOH functional group borne by the ethylenically unsaturated starting compound.